Materials such as GaN, AlN, and InN that are nitride semiconductors, and mixed crystals of such nitride semiconductors, have a relatively wide band gap and may thus be used for high-output electronic devices, short-wavelength light emitting devices, or the like. For example, the nitride semiconductor GaN has a band gap of 3.4 eV, which is wider than the band gap of 1.1 eV for Si, and the band gap of 1.4 eV for GaAs.
The high-output electronic devices may include a FET (Field Effect Transistor), particularly a HEMT (High Electron Mobility Transistor). The HEMT that uses the nitride semiconductor may be used for a high-output, high-efficient amplifier, a high-power switching device, and the like. Particularly, in the HEMT using a AlGaN electron supply layer and a GaN channel layer, a piezo polarization occurs in AlGaN due to a distortion caused by the difference in lattice constants of AlGaN and GaN, and a high-density 2DEG (Two-Dimensional Electron Gas) is generated. For this reason, this HEMI may operate at a high voltage, and may be used for a high-efficient switching device, and a high-voltage power device employed in electric cars and the like.
Usually, the 2DEG exists in a region immediately under a gate, even in a state in which no voltage is applied to a gate electrode or the like, and the device that is fabricated becomes a normally-on device. Hence, in general, in order to fabricate a normally-off device, a part of a nitride semiconductor layer in the region where the gate electrode is formed is removed by etching, in order to reduce the distance between a channel and the gate electrode and form a gate recess structure.
From the point of view of improving characteristics of the device or the like, a gate insulator layer may be formed between the gate electrode and the nitride semiconductor layer. Aluminum oxide (Al2O3) formed by ALD (Atomic Layer Deposition) is suited for such a gate insulator layer, and may be regarded as a promising material for use as the gate insulator layer, because the withstand voltage may be 10 MV/cm t0 30 MV/cm and high.
A field effect compound semiconductor device is proposed in a Japanese Laid-Open Patent Publication No. 2002-359256, for example.
Residue of impurities originating from a source gas may exist in an aluminum oxide layer that is formed by the ALD. More particularly, in the aluminum oxide layer formed by the ALD, hydroxyl group (OH group) may remain in an aluminum hydroxide (Al(OH)x) state, and the OH group may act as an electron trap. Hence, this electron trap may cause a threshold variation of the gate voltage and prevent the fabrication of the normally-off device.
On the other hand, the aluminum oxide layer may be formed on the nitride semiconductor layer, such as GaN. However, GaOx may be formed at an interface between the GaN and the aluminum oxide layer, and the GaOx may also act as an electron trap. Accordingly, this electron trap may cause the threshold variation of the gate voltage and similarly prevent the fabrication of the normally-off device.
When the gate insulator layer is formed on the gate recess structure, the step coverage may become relatively poor. However, in the case of the aluminum oxide layer formed by the ALD, a satisfactory step coverage may be obtained.